Sedimentary evolution of the Ediacaran Yangtze platform shelf (Hubei and Hunan provinces, Central China)

Sedimentary Geology 225 (2010) 99–115

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Sedimentary Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s e d g e o

Sedimentary evolution of the Ediacaran Yangtze platform shelf (Hubei and Hunan provinces, Central China) Elodie Vernhet a,⁎,1, John J.G. Reijmer b a b

Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany VU University Amsterdam, Faculty of Earth and Life Sciences (FALW), Dept. of Sedimentology and Marine Geology, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

a r t i c l e

i n f o

Article history: Received 29 February 2008 Received in revised form 21 January 2010 Accepted 26 January 2010 Available online 1 February 2010 Communicated by B. Jones Keywords: Yangtze platform China Doushantuo Formation Stratigraphical correlation Ediacaran

a b s t r a c t The development of the Ediacaran Doushantuo Formation (635–551 Ma) on the Yangtze platform is studied to analyse the environmental changes on this platform accompanying the Ediacaran bioradiation. In this study we will present a detailed facies analysis and propose a tentative stratigraphic analysis of seven selected sections that are located predominantly on the platform shelf situated in the Hubei Province. The Doushantuo Yangtze platform displays two major depositional environments: a rimmed carbonate platform and an open shelf. The shift from one depositional model to another may be related to the paleotopography in horsts and grabens inherited from previous extensional movements. The strata observed in the shelf deposits of the Doushantuo Formation are organised in three stacked facies successions. These shallowingupward facies successions may be correlatable with other Ediacaran sections worldwide. © 2010 Elsevier B.V. All rights reserved.

1. Introduction

1.1. Location and geological setting

The Doushantuo Yangtze platform is one of the most unique locations in the world to study the depositional conditions and paleoenvironmental and paleogeographic setting of the Ediacaran bioradiation. The exceptionally well-preserved fossil record was documented extensively in a series of paleontological studies (Chen et al., 1991, 1999; Babcock and Zhang, 2001; Babcock et al., 2001; Yin et al., 2001; Xiao et al., 2002; Hou and Bergstrom, 2003; Chen et al., 2004; Hou et al., 2004; Shu et al., 2004; Yin et al., 2004). However, only a few studies addressed the sedimentary evolution of the Doushantuo Yangtze platform. Our study may help to understand the environmental conditions that accompanied the Ediacaran bioradiation. The detailed facies analysis presented in this study will provide information on the lateral changes of the depositional environments. The study of the facies stacking may help to correlate the sections across the platform and thus will shed new light on the evolution of the Yangtze platform through Doushantuo Formation times. Moreover, the discussion on the factors controlling the facies stacking may help to constrain the environmental variations prior to the Ediacaran bioradiation.

The studied sections of the Doushantuo Formation are located in the northern Hunan and central Hubei provinces (Fig. 1). They represent the shallow-water environment of the Yangtze platform during the Ediacaran (Fig. 2A). The present-day Yangtze platform is located between the Qin-Lin orogeny to the North and the Cathaysia suture to the Southeast. The North China craton to the north collided with the Yangtze craton during the early Triassic (Hsü and Chen, 1999), while the Cathaysia block, positioned in the SE, formed a block with the Yangtze craton during the Silurian (Fig. 2B). Large-scale fault-bounded basins that were filled with continental facies during the Cretaceous to some extent, mask the outcrops in which the Proterozoic sediments occur. The preserved Proterozoic–Paleozoic strata are several kilometers thick and only show moderate to large-scale folding. After the break-up of the Rodinia supercontinent, the Ediacaran platform has evolved into a passive margin since the late Cryogenian.

⁎ Corresponding author. E-mail address: [email protected] (E. Vernhet). 1 Present address: Université des Antilles et de la Guyane. Campus Fouillole. Département de Géologie. F-97159 Pointe-à-Pitre. Guadeloupe (FWI). 0037-0738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2010.01.005

1.2. Stratigraphic succession Stratigraphic nomenclatures given to the Ediacaran successions in central China vary regionally, mostly because of lithological and facies changes. Erdtmann and Steiner (2001), Wang and Li (2003), and Zhu et al. (2003) proposed a correlation between different regional formation names for the Ediacaran Yangtze platform (Fig. 3). The sediments of the Cryogenian Nantuo Formation diamictites are interpreted as tillites that are overlain by the Ediacaran succession. However, the glacial character

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Fig. 1. Location of the study area. Triangles denote location of measured sections.

of the Nantuo Formation diamictites is still discussed (Eyles and Januszczak, 2004). Its precise age is also debated: Evans et al. (2000); Wang and Li (2003) argued for a Sturtian age (approx. 750 Ma, Frimmel et al., 2002), whereas Jiang et al. (2003), Chen et al. (2004), Dobrzinski et al. (2004), Zhou et al. (2004), and Condon et al. (2005) proposed that the Nantuo diamictites were deposited during the Marinoan glaciation (approx. 635 Ma,). The thickness of the Nantuo Formation on the southern Yangtze platform ranges from 0 to more than 2000 meters. Latter maximum thickness is reported from the northern Guangxi province (Wang and Li, 2003; Zhang et al., 2003). These diamictites attracted renewed scientific interest after the publication of the “Snowball Earth” theory (Hoffman et al., 1998; Hoffman and Schrag, 2000; Hyde et al., 2000; Runnegar, 2000; Hoffman and Schrag, 2002;

Donnadieu et al., 2004). An approximately six meters thick succession, the so-called cap carbonate, overlies the diamictites and forms the basal unit of the Ediacaran Doushantuo Formation. These carbonates show unusual sedimentary structures as vertical tube-like structures (Nogueira et al., 2003), very large-scale wave ripples (Allen and Hoffman, 2005), aragonite fans (Sumner, 2002) and a negative δ13C isotope anomaly (Knoll et al., 1993; Saylor et al., 1998; Corsetti and Hagadorn, 2000; Shen et al., 2005). The Nantuo diamictites and the cap carbonate are present throughout the central and southern Yangtze Platform. The Doushantuo Formation on the Yangtze Platform continues with black shales that were deposited below wave base, shallow-water carbonates, and regionally traceable phosphorite horizons (Zhu et al., 2003). The contact between the sediments of the Doushantuo Formation and the overlying Dengying Formation is marked by the first occurrence of light grey dolomitized limestones. Only a few sections, however, show this contact (Maoping and Wuhe sections in the Hubei Province; Yangjiaping, and Zhongling sections in the Hunan Province). Towards the southeast, the dolomitized limestones of Dengying Formation grade downdip into silicified black shales of the Liuchapo Formation (Fig. 3), which are widespread in the Yangtze platform slope and basinal environments of central Hunan and eastern Guizhou provinces. 2. Facies description The base of the Doushantuo Formation consists of a 4 to 6 m thick dolomite sequence (capdolomite), which overlies the Nantuo diamictites. These dolomites and diamictites are not the focus of our study and are only used as time markers to facilitate correlation between the sections studied. Within the overlying deposits of the Doushantuo Formation ten sedimentary facies were identified, which are grouped into four facies associations that include low-energy shallow-water,

Fig. 2. A. Paleoenvironmental reconstruction of the study area during Doushantuo time (∼590–540 Ma) (after Steiner, 2001). B. Tectonic blocks forming present-day eastern China. North China craton collided with Yangtze craton during the Permian/Trias, while Cathaysia collided with this structure during the Silurian (after Wang and Mo, 1995).

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Fig. 3. Schematic stratigraphic columns and correlation diagram for the Ediacaran and Cambrian system of Hunan and Hubei Provinces. Data from Brasier et al., 1994.

high-energy intertidal, shallow subtidal and slope/basin facies associations. Their characteristics are summarized in Table 1. 2.1. Low-energy, shallow-water, facies association 2.1.1. Description Four facies (1–4) are recognized in this facies association (see Table 1). Facies 1 comprises 5 to 30 cm thick, thinly laminated carbonaceous shales that are interbedded with (a) phosphoritic or dolomitic wackestones and (b) five to ten cm thick, fine sand-sized intraclastic packstones to grainstones. Their bedding is planar and laterally continuous. The thickness of this facies interval varies between two and several tens of metres. In thin section, the wackestones show poorly sorted, detrital dolomite grains or opaque crystals in a micritic matrix. A few euhedral crystals of dolomite are present. Thin, opaque laminations can also be observed. Facies 1

outcrops in the Zhancumping and Xiaofenghe (Hubei province) and in the Zhongling and Yangjiaping sections (Hunan province). Facies 2 occurs in the Zhancumping section in the Hubei province and consists of a 40 cm thick interval of laminated phosphoritic wackestones with mm- to cm-sized, phosphorite micrite or dolomite micrite intraclasts (Fig. 4). Beds are generally 10- to 15-cm-thick and are laterally continuous. The beds show laminations that are laterally continuous. In thin section, these laminations appear as opaque organic-rich levels. Their thickness varies around tens of μm. The rare mm-sized grains consist of opaque and rounded clasts. Few grains are cm-sized laminated dolomitic micrite clasts. Phosphorite micrite (brown matrix) dominates in the thin section. Facies 3 consists of thin-bedded (2 to 10 cm), white dolomite micrite interbedded with (a) black phosphorite micrite and (b) very thin-bedded grainstones that have a very local appearance. The dolomite micrite generally constitutes more than 60% of the total rock

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Table 1 Summary of facies from the Doushantuo Formation shelf. Facies association

Facies

Low-energy Facies 1 shallow-water

Facies 2

Facies 3

Facies 4

High-energy, Facies 5 shallow-water

Description

Components

Grains description

Bedding type

Sedimentary structures

Vertical facies Interpretation organisation

Laminated shale/ mudstone/wackestone interbedded with sand-sized packstone. Biolaminated phosphorite

(Calcareous) shale, medium sand-sized packstone Phosphorite

Thin-bedded, laterally continuous

Thin parallel laminations due to biomats

5, 7

Lagoon or lowenergy intertidal

cm-thick, laterally continuous

Biolaminations.

7

Lagoon margin with emergence period

Phosphorite micrite interbedded with evaporitecontent dolomite Phosphorite grainstone interbedded with thinly laminated dolomite

Phosphorite, dolomite micrite, evaporite Phosphorite, dolomite

mm-sized intraclasts, dolomite and opaque crystals mm-sized dolomite and phosphorite intraclasts. Biolaminated clasts. Rare phosphorite intraclasts

Lenticular bedding

Evaporite-induced convolutes

2

cm-thick, lenticular bedding

Biolaminations in dolomite

1, 3

Conglomerate with cm-to-msized mud clasts and ooliths

Dolomite, phosphorite

mm-sized, very well-sorted phosphorite intraclasts cm-sized, phosphorite or dolomite intraclasts, ooliths, oncoliths mm- to cm-sized phosphorite and dolomite intraclasts and ooliths Medium sandsized dolomite intraclasts

Peritidal, lagoon margin. Evaporitic conditions Peritidal, lagoon margin

cm- to m-thick limited lateral extend

Mass deposits, erosive base

6

Debrites

cm- to dmthick limited lateral extend

dm-scale trough and planar crossbedding

5

Wave-influenced environment, tidal channels

cm-to-dmthick bedding, laterally continuous dm-thick bedding, crossstrated, laterally discontinuous Thin-bedded, laterally continuous

Plane horizontal bedding, cmscale trough crossbedding

1, 4, 8

Subtidal above fair weather wave base

Fine plane horizontal bedding. Reactivation surfaces

7

Sand bars

Thin parallel laminations due to discrete change in grain size

1, 4, 7

Basin and slope with turbidite deposits

Thin-bedded, laterally discontinuous

Thin parallel laminations due to discrete change in grain size, slump folds. Variable in olistoliths

9

Slope with turbidite deposits and olistostromes

Facies 6

Grainstones with trough Dolomite, or planar crossbedding phosphorite, calcitic cement

Mediumenergy, Facies 7 shallow-water

Dolomitised Medium sand-sized limestone wackestone/packstone with plane parallel or trough crossbedding Cross-strated grainstone Dolomitised limestone

Facies 8

Low-energy, deep-water

Facies 9

Shales interbedded with phosphorite and/or wackestone to packstone Facies 10 Shales interbedded with wackestone to packstone. Slump folds, olistoliths

Medium sandsized dolomite intraclasts

(Calcareous) shale, phosphorite, dolomite (Calcareous) shale, dolomite. Variable for olistoliths.

volume and is organised in irregular-shaped, lenticular, thin beds with a restricted lateral continuity. The dolomite micrite lenses show irregular edges and contain cm-sized convolute-like deformation structures, which may be caused by the interbedded grainstones (Fig. 5A). The phosphorite micrite layers consist of continuous cm-thick beds on outcrop scale that locally are disrupted by irregular-shaped white dolomite micrite lenses. The grainstones consist of very fine sand-sized, well-sorted, black rounded phosphorite grains (Fig. 4) embedded in a white dolomitic cement. In thin section, the white dolomite micrite consists of equant, well-shaped dolomite crystals, a few μm-thick. The phosphorite micrite shows irregular-shaped patches of light brown-coloured apatite surrounded by a thin dark brown fringe. Opaque, μm-sized minerals are abundant. The grains in the grainstone consist of rounded, brown-coloured apatite intraclasts or apatite oolites. Facies 3 is present in the Zhancumping section in the Hubei province. Facies 4 consist of irregular, thin-bedded (1–3 cm) grainstones and comprises, at least, 90% of the total rock volume (Fig. 4). The grains consist of black, rounded, well-sorted, mm-sized (phosphorite) micrite intraclasts. A white matrix can, rarely, be present between the grains. No sedimentary structures have been observed in the grainstone beds. The grainstones are interbedded with 2 to 5 mm

thick, thinly laminated, irregular-shaped white dolomite micrite layers. In thin section, the thin laminations present in the dolomite micrite consist of black opaque, irregular organic-rich sediments. Rare, euhedral, dolomite crystals are present between the organicrich beds. The grainstone grains are made up of apatite intraclasts that show a clear zonation in which the core of the individual grains is darker than the edge. A thin fringe of cement may separate the grains, but commonly, grains are in direct contact. Locally, grains are not regularly dispatched and an apatite matrix may be present. Facies 4 occurs in the Zhancumping section in the Hubei province. The facies described are present in the middle part of the Zhancumping section and succeed each other as follows: facies 4 at the base, followed by facies 3 and facies 2 on the top, which is overlain by facies 7. Facies 4 at the base of this succession overlies the shale of the facies 9 (see following chapters). 2.1.2. Interpretation The finely laminated carbonaceous shales of facies 1 represent a low-energy environment, dominated by suspension-settling processes. The intercalated minor sand-grained packstones to grainstones most likely represent high-energy events (storm events?) that transported coarse-grained sediments into a mud-dominated

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Fig. 4. Outcrop (pencil = 14 cm), hand-sample (black and white scale = 5 cm.), and thin-section pictures (black line = 400 μm) of interpreted low-energy, shallow-water facies 2, 3, and 4. Phosphorite and evaporite-content dolomite dominate the lithology of these facies, which have been deposited on the edge of a lagoon. F.A.: Facies Association. Fac.: facies number. Pictures from Zhancumping section, Hubei province.

protected area. The organic-rich laminations and the black colour of deposits may suggest an oxygen-deficient depositional environment. The facies 1 deposits grade up to facies 4 deposits. The micritedominated facies 1, 2 and 3 and the sedimentation of organic-rich layers in facies 2 and 4 argue for a low-energy depositional environment. The convolute-like structures (Fig. 5A) in facies 3 may be formed by overpressure that occurs within water-rich sediments when they become overlain by other sediments (Reineck and Singh, 1975; Reading, 1989) or by the precipitation and dissolution of evaporites (Blatt, 1992). The absence of organic-rich layers in facies 3 – although they are present in the underlying facies 4 and the overlying facies 2 – may be due to the presence of evaporite that may block the development of organic matter (Walker and James, 1992). In the sediments of facies 4, the lateral changes of phosphorite grains into phosphorite micrite indicate that the grains may be formed by direct precipitation of apatite as rounded grains (Trappe, 1998). Therefore, these grainstones not necessarily indicate deposition in an agitated sedimentary environment. The absence of sedimentary structures in the grainstones may support this interpretation. In summary, the depositional environment of facies 2, 3, and 4 most likely represents sediment deposition in a protected shallow-

water environment with possible evaporation processes and local water oversaturation. Such a setting can be found in a peritidal environment fringing a lagoon/intrashelf basin under warm climate. The grainstone intercalations in facies 1 likely represent storminduced sediment layers. The depositional environment shows a shallowing-upward sequence when moving from facies 4 to facies 2 (Fig. 4). The facies 2 represents the most shallow part of the lagoon, located in the zone of water level variation. 2.2. High-energy, shallow-water facies association 2.2.1. Description Facies 5 consists of decimeter to meter thick-bedded, grainsupported conglomerates (Fig. 5B). The matrix consists of well-sorted, mm-sized grains, predominantly black phosphorite micritic ooids. The pebbles are made up of large rounded phosphorite or mudstone/ wackestones, 3 to 10 cm in size. Locally, meter-scale mudstone/ wackestone boulders are present. Stylolites are abundant. An erosive surface marks the base of the conglomerates. These conglomerates are commonly interbedded with cm to dm thick, bedded mudstone to wackestone. In thin sections, the ooids may be oolites type “α” or

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Fig. 5. Outcrop and hand-sample pictures of interpreted facies. A. Detail of facies 3 showing convolute-like deformations due to the presence of evaporite (hand samples, Zhancumping section; Hubei province). B. Trough-crossbedded grainstones of facies 6 with an overlying grain-supported conglomerate with cm-sized mud clasts corresponding to facies 5 (Yangjiaping section; Hunan province) (part of pencil for scale: 3 cm). C. Crossbedding in phosphoritic grainstone of facies 6 present in a tide channel (Zhongling section, Hunan province).

coated grains. The oolites show a nucleus consisting of dolomite crystals or micrite fragments, which is surrounded by several rings of brown apatite micrite. The coated grains present a single ring of brown apatite micrite. Their nuclei are formed by dolomite crystals or brown apatite micrite. The type of ooids may change depending on the individual conglomerate beds. In the same conglomeratic bed, the ooids embedded in sparite matrix appear similar in size and in type. Facies 6 consists of 10 to 30 cm thick, sand-sized grainstones interbedded with 1 to 5 cm thick-bedded mudstone to wackestone. In this facies dm-sized trough or oblique planar crossbedding is present (Fig. 5D). The grains are well-sorted, mm-sized and consist of phosphorite or dolomite micritic intraclasts or ooids in which phosphorite coated grains are common. In addition, elongated cm-sized mudstone “chips” frequently occur in the grainstones. Facies 5 and 6 are found in the Zhongling and the Yangjiaping sections in the Hunan Province.

2.2.2. Interpretation The origin of mudstone/wackestone pebbles is uncertain. However, the sedimentation of conglomerates may result in reworking of the interbedded thin mudstone/wackestone layers to form the mud pebbles and boulders.

The conglomerates of facies 5 most likely represent gravity deposits that were deposited in the vicinity of an ooid sand shoal. Unfortunately, the outcrop conditions do not allow a detailed evaluation of the lateral extent and the angle of repose of these conglomerates. Hence, the nearby presence of an ooid sand shoal is inferred from the presence of ooids in the conglomerates. The depositional environment of this conglomerate may have been deeper than 4 m, the maximum water depth for ooid formation (Scoffin, 1987; Greensmith, 1989; Tucker and Wright, 1990). Gravity-related mass movements remobilized sediments from ooid sand shoals and deposited the sediments in a lowenergy environment dominated by suspension-settling (micrite beds). Storms on the ooid shoal or sediment readjustment processes of its slope may have induced the redeposition of the sediments. Trough crossbeddings (facies 6) are common and suggest the migration of ripples, while oblique planar crossbeddings (facies 6) suggest a high-energy regime in the vicinity of (tide?) channels (Mueller et al., 2002). The interbedded thin micrite layers indicate suspension-settling-dominated periods. Thus, facies 5 and 6 most likely represent deposition in an intertidal/subtidal environment in which periods of suspension-settling sedimentation alternate with periods with high-energy currents. These deposits therefore might represent overbank deposits that can be found near (tide?) channels (Reineck and Singh, 1975).

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2.3. Medium-energy, shallow-water facies association 2.3.1. Description This facies association includes two medium-grained calcareous facies 7 and 8 with current-related sedimentary structures. The thickness of the facies 7 intervals ranges from one to a few tens of meters. The beds are dm-thick, planar and laterally continuous. Facies 7 comprises sand-sized dolomitised packstone to grainstone with plane horizontal laminations or cm-scale trough cross laminations (Fig. 6A). A few mm-thick plane horizontal laminations show normal grading in grain size. The grains consist of micrite peloids. In thin section, the grainstone cements consist of euhedral dolomite crystals. Facies 7 dominates the sedimentation of the shelf and is present in almost all studied sections of the Yangtze platform shelf. Facies 8 represents 2 to 3 m thick intervals, which are not laterally continuous, of 30-to-100-cm thick-bedded dolomitized sand-sized grainstones. Beds show an erosive base and contain lenticular-shaped

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m-scale cross-stratifications (Fig. 6B). Thin plane horizontal bedding with intra-formational erosive surfaces (Fig. 6C) have been observed in these lenticular beds. Diagenetic dolomites locally obscure the initial sediment facies. Facies 8 is located in the Zhancumping and the Xiaofenghe sections in the Hubei province. 2.3.2. Interpretation The presence of cross-bedding structures in facies 7 indicates a depositional environment with periodic high-energy currents. The lateral continuity of the facies 7 deposits argues in favour of an extended high-energy environment such as a shallow-water open shelf. Facies 8 probably represent sand bars formed by coastal currents that reworked the sediments of the shelf. Actually such sand bars form between 5 and 70 m waterdepth (Reynaud et al., 1999; Trentesaux et al., 1999). Only the Zhancumping and the Xiaofenghe sections comprise facies 8. This interval has a limited lateral extent : it is impossible to correlate these deposits along the entire shelf. Thus, the sediments of Facies 7 and 8 were deposited in an agitated

Fig. 6. Medium-energy, shallow-water facies association. A. Facies 7 shows wackestone/packstone with plane-parallel bedding (Zhongling section, Hunan Province) (black line for scale: 8 cm). B. Facies 8 is cross-bedded grainstones (in circle, geologist hammer for scale). C. Detail from facies 8 showing plane bedding and reactivation surface. Pictures B and C from Xiaofenghe section, Hubei Province (black line for scale: 1 cm).

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sedimentary environment. The winnowing of sediment and the planar organisation of the beds suggest a shallow subtidal platform environment.

2.4. Low-energy, deep-water facies association 2.4.1. Description The low-energy, deep-water facies association includes facies 9 and 10. These facies may be tens of meters thick and are laterally persistent. Facies 9 comprises 5 to 20 cm thick, thinly laminated black (silicified) shales. The black shales are locally interbedded with thinbedded phosphorites and/or very fine-grained grainstones. Beds are laterally continuous and planar. Facies 9 covers the cap carbonate that occurs at the base of the successions in all studied sections. Facies 10 consists of thin-bedded, thinly laminated (silicified) black shales. The shales are interbedded with very fine-grained grainstones. Meter-scale slump folds (Fig. 7A) disturb the bedding. Locally, sediments show wedge-shaped bedding (Fig. 7B). Facies 10 can be found in the Maoping, the Neo-Tiajiayuanxi, and the Wuhe sections (Hubei Province). In the Maoping section, a dolomitized limestone interval is present within the black shales.

2.4.2. Interpretation The black shales of facies 9 represent a low-energy depositional environment dominated by suspension-settling sedimentation. In absence of marker fossils, the interpretation of these deposits is based on the facies succession itself and the lateral evolution. The cap carbonate is used as a stratigraphic marker. For the facies 10, a slope environment is suggested by the gravity-related sedimentary structures such as slump folds at various scales and discrete discontinuities (Coniglio and Dix, 1992). The lateral association of facies 9 within a slope environment (facies 10) argues for sedimentation in a relatively deep basinal environment in which this shale was deposited under anoxic conditions. The thin-bedded grainstones may represent gravity deposits resulting from sediment redeposition events. Facies 9, which directly overlies the cap carbonate, is commonly inferred to have been deposited during a relative high sea-level stand following the Marinoan deglaciation (Allen and Hoffman, 2005). Thus, the sediments of facies 9 and 10 can be interpreted as sediments deposited in a slope to basin environment.

3. Depositional environments The facies analysis suggests that the sedimentation on the southern Yangtze platform during the Ediacaran Doushantuo interval took place within two depositional environments: a rimmed platform and an open shelf platform (Fig. 8).

3.1. Large rimmed platform The important change and diversity of facies exclude a ramp morphology for the Ediacaran Yangtze platform. Within a rimmed platform morphology, sedimentation in a back rim environment is characterised by low-energy deposits dominated by suspension settling, while the rim protects a back-rim, low-energy basin and the peritidal environments from the open ocean (Tucker and Wright, 1990). Low-energy facies (facies 1, 9, and 10) characterise the first stage of the Yangtze platform development. A rim facies with domal, columnar microbialites or “in situ” sand shoals have not been observed in outcrop. The rim, however, is inferred from the presence of suspension settling-dominated facies and from the presence of shallow-water intrashelf basins located on the platform margin (Vernhet et al., 2006; Vernhet, 2007).

Fig. 7. Facies from low-energy, deep-water facies association. A. Large-scale folded black limestone (Maoping section, Hubei Province). B. Wedge-shaped terminations (white arrows) in packstone (Neo-Tianjiayuanxi, Hubei Province) (black line for scale = 30 cm.).

3.2. Open shelf platform The coarse-grained, high- to medium-energy facies (facies 5 to 8), that were deposited during the second stage of the Yangtze platform development, argue for sediment deposition on an agitated platform. This interpretation of an open-shelf platform environment is also suggested by the presence of the packstones with sedimentary structures (facies 7) that were deposited by coastal currents and/or wave action (Jones and Desrochers, 1992). Locally, high-energy currents reworked sediments from the shelf and formed sand bars (facies 8) and ooid shoals moving on the shelf (Meng et al., 1997; Reynaud et al., 1999; Trentesaux et al., 1999). The ooid shoal as inferred from the conglomerates of facies 5 and few deposits of facies

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Fig. 8. Depositional environmental models. A. Rimmed epeiric shelf model. The large dimension of the shelf allowed the evolution of independent systems such as the intra-shelf basin in Maoping section, Hubei province. B. Open shelf model. In the absence of a rim the sea entered the shelf with substantial energy. The currents formed the grainy facies.

6, may have been deposited at the edge of the platform and probably were formed by currents and/or waves (Chen et al., 2002). 4. Vertical facies organization The facies can be grouped into stacked sediment successions types A and B (Figs. 9, 10). This facies stacking appears to have a regional extension and to record a systematic organisation with a facies progradation upward. The proposed vertical stacked sediment successions type A and type B show lateral variations that are numbered from 1 to 3. (Figs. 11, 12).

4.1. Type A vertical facies succession This first type of vertical facies stacking may be constrained by the sea level highstand following the worldwide Marinoan ice-sheet melting (Wang et al., 1998; Zhu et al., 2003). The type A vertical facies successions show a thick basal (approx. 20 m) interval of black shales (facies 9 and 1) overlying the cap carbonate. Only the uppermost part of these sediment piles show lateral variations according to the section location on the platform. These lateral variations correspond to a peritidal environment evolving into a restricted intra-shelf basin with gravity-related mass movements (Fig. 11) on the basin edge. In

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Fig. 9. Lateral evolution of stacked facies succession type A in a prograding depositional environment.

the Zhancumping section (Hubei Province), facies 4, 3, and 2 form the top of this facies succession type A1 (Figs. 9, 11). In the Xiaofenghe, the Wuhe and the Maoping sections (Hubei Province) and in the Zhongling and the Yangjiaping sections (Hunan Province) which present vertical sedimentary sucession type A2 and A3 respectively, facies 1 overlay facies 9.

4.2. Type B vertical facies succession Important lateral facies variations characterise the type B vertical sediment succession (Fig. 12). In Maoping, Wuhe and Neo-Tianjiayuanxi sections (Hubei Province), black shales dominate the sedimentary record (facies 9) (type B1). They are interbedded with thin-bedded packstones to grainstones (facies 10). Slump folds are common. A thick interval of limestones with large slump folds marks the top of this sequence (facies 10). In Zhancumping, Xiaofenghe, Wuhe, Neo-Tianjiayuanxi sections (Hubei Province), and Zhongling section (Hunan Province), the base of the sedimentary succession type B2 shows medium-energy sediments (facies 7, 8) deposited in a shallow-water, subtidal environment. These medium-energy deposits evolve upward in mudstones or interbedded black shales and mudstones (facies 1) characteristic for sediment deposition in a lagoonal environment. In Zhongling and Yangjiaping sections (Hunan Province) the base of the vertical facies succession type B3 shows high-energy, shallow-water environment with crossbedded grainstones and associated conglomerates (facies 5, 6) overlain by (Dengying Formation) mudstones to wackestones (facies 1)(Figs. 13 and 14).

5. Discussion 5.1. Evolution of the Yangtze platform geometry Two geometries for the platform have been identified: (1) a rimmed, shallow-water platform with an important diversity of depositional environments and (2) a wave-dominated open shelf (Fig. 15). According to Wang and Mo (1995) the Cryogenian Nantuo Formation diamictites represent the last stage in the rifting history of the Yangtze platform following the breakup of the Rodinia supercontinent. The diamictites did not fill up the “horst and graben” paleobathymetry developed by the rifting process (Vernhet, 2007). Hence, the basal sediments of the Doushantuo Formation Yangtze platform (until SB-1) were deposited on this irregular surface and developed a very diversified, “mosaic”-type (Pratt and James, 1986), facies organisation. The intrashelf basins located near the Maoping section (Hubei Province) may represent paleograben. Evidence for the presence of shallow-water enclosed intrashelf basins on the platform margin has been described in Vernhet et al. (2007). The shallowwater basins developed in fault-bounded depressions and the inferred rim on the residual relief of horsts. The presence of these basins at the platform margin argues for only a thin layer of water that locally may have covered the Yangtze platform (Vernhet et al., 2007). The infilling of the residual relief during the relative rise in sea level may explain the evolution of the Yangtze platform from a rimmed platform geometry to a wave-dominated open shelf geometry. The predominance of shallow subtidal coarse-grained facies with high-energy sedimentary structures argues for an average water depth above the storm wave base (100 m water depth).

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Fig. 10. Lateral evolution of stacked facies succession type B in a prograding depositional environment. FWB: fair weather wave base.

A transitional period between the rimmed platform geometry and the open shelf geometry has not been identified in outcrop. A rim may have compensated the variation of relative sea level. Thus, the reflooding of this steep-sided topography may have induced the sudden inundation of the platform, which may explain the absence of recorded transitional geometry. 5.2. Correlation with other Ediacaran platforms worldwide In the literature, the Doushantuo Formation has been subdivided into four members according to the lithology: (a) limestone (cap carbonate), (b) shale, (c) limestone, and (d) shale (Erdtmann and Steiner, 2001; Zhu et al., 2003). The shales overlying the cap carbonate have been interpreted to represent sedimentation during a highstand in sealevel following the Marinoan deglaciation (Wang et al., 1998; Zhu et al., 2003). The overlying limestones represent the sea-level fall or lowstand and the terminal black shale of the Doushantuo Formation another rise in sea level. In the absence of identifiable fauna, the depositional environment of the Ediacaran black shales on the Yangtze platform remained uncertain and the environmental interpretation of these deposits was based on the fact that black shales usually represent deep-water deposits (Wang et al., 1998; Wignall and Newton, 2001; Jiang et al., 2003). Shales corresponding to a highstand in sea level covered the shelf and can be correlated over the entire platform and between different platforms worldwide (Jiang et al., 2003; Allen and Leather, 2006; Allen, 2007). During major sea-level rises in which black deep-water shales may be deposited on the platform, all existing shallow-water facies should be pushed back towards the edge of the basin. Own data and data from literature (Guizhou Province's Bureau of Geology and Mineral Resources, 1987; Hubei Province's Bureau of Geology and Mineral

Resources, 1990; Hunan Province's Bureau of Geology and Mineral Resources, 1988) allow to draw paleoenvironmental maps showing the limits between the shelf (limestones, shallow-water facies), the slope (limestones, black (silicified) shales with gravity-related deformation) and the basin (black (silicified) shales) during the Doushantuo and the Dengying/Liuchapo Fm. These maps show that globally the slope remained located in Middle Hunan and Eastern Guizhou Provinces during the Doushantuo and the Dengying/ Liuchapo Fm (Fig. 16). The slope environment did not record a continent orientated northward shift. Thus, the black shales on the shelf are not related to a highstand following a sea level rise. Hence, the Doushantuo Fm black shale most likely were deposited in a protected environment on the shelf, which was dominated by suspension settling sedimentation. The absence of water displacement induced the anoxic conditions needed to preserve the organic matter. As a consequence the black shales of the Doushantuo Fm have a limited lateral extent and may not serve as stratigraphical marker beds. Recent work on clay minerals shows that a part of the intra-shelf basins on the Yangtze platform should be nonmarine (Bristow et al., 2009). The low-frequency shallowing-upward facies successions described in this article have been already identified on other Ediacaran platforms (Jiang et al., 2003; Allen and Leather, 2006; Allen, 2007). However, the new depositional environment proposed for the black shales should induce new interpretations about these worldwidecorrelatable sedimentary cycles. 6. Conclusions This study provides evidence to better understand the development of the Yangtze platform during the Doushantuo Formation. The

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Fig. 11. Description of shallowing-upward, facies stacking successions type A. These facies successions have been deposited directly after the Marinoan deglaciation, on the top of the tillites. The black shales may represent the sea-level highstand after deglaciation. The presence of peritidal facies on the top of facies succession type A1 argues for a shallow-water depositional environment for at least the upper part of these black shales.

facies analysis showed that the Yangtze platform presented two successive geometries: a shallow-water rimmed platform with a “mosaic” type of facies distribution and a wave-dominated open shelf with an average water depth of about 30 m. Sediment stackings are organised in shallow-upward facies succession that can be correlated throughout the entire shelf. The shales and mudstones that occur in the middle of the successions studied, but are absent in the Maoping and Wuhe sections in Hubei Province, are interpreted as sediments deposited in a restricted shallow-water environment and likely result from a drop in relative sea level.

Acknowledgements Financial support for this study was given by the German Science Foundation (DFG) and the National French Science Foundation (NSFC). The first author thanks Prof. Zhu and Prof. Zhang (both at Paleontology Institut of Nanjing, PR China) for the organisation of joint fieldwork. Prof. B.D. Erdtmann (Technischer Universität Berlin, Germany), Prof. C. Heubeck (Freie Universität Berlin, Germany), Prof. M. Zhu, (Paleontology Institut of Nanjing, PR China) and Prof. J. Zhang

(Paleontology Institut of Nanjing, PR China) are acknowledged for valuable discussions. Prof. H. Chellai (Cadi Ayyad University, Marrakech, Morocco) is thanked for his constructive comments on a first draft of the manuscript. This manuscript forms part of the first authors Ph.D. Dissertation at the FU Berlin. References Allen, P.A., 2007. The Huqf supergroup of Oman: basin development and context for neoproterozoic glaciation. Earth-Science Reviews 84, 139–185. Allen, P.A., Hoffman, P.F., 2005. Extreme winds and waves in the aftermath of a Neoproterozoic glaciation. Nature 433, 123–127. Allen, P.A., Leather, J., 2006. Post-Marinoan marine siliciclastic sedimentation: the Masirah Bay Formation, Neoproterozoic Huqf Supergroup of Oman. Precambrian Research 144, 167–198. Babcock, L.E., Zhang, W., 2001. Stratigraphy, paleontology, and depositional setting of the Chengjiang lagerstätte (lower Cambrian), Yunnan, China. In: Peng, S.-C., Babcock, L.E., Zhu, M.-Y. (Eds.), Cambrian system of South China: Palaeoworld, vol. 13, pp. 66–86. Babcock, L.E., Zhang, W., Leslie, S.A., 2001. The Chengjiang biota: record of the early Cambrian diversification of life and clues to exceptional preservation of fossils. GSA Today 11, 4–9. Blatt, H., 1992. Sedimentary petrology. W.H. Freeman and Company, New York. 514 p. Brasier, M., Cowie, J., Taylor, M., 1994. Decision on the Precambrian–Cambrian boundary stratotype. Episodes 17, 3–8.

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Fig. 12. Description of shallowing-upward facies stacking successions type B. These series show the evolution of an open shelf during a drop in sea level.

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Fig. 15. Block-diagram showing the sedimentary evolution of the Yangtze platform during the Doushantuo Formation.

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Fig. 16. Paleogeographical reconstruction of the shelf margin of the Yangtze platform compiled from literature and own data. A. During Doushantuo Formation. B. During Liuchapo/ Dengying Formation. Note the permanent location of the slope environment.